Dual cavity bandpass filter



. )June 2, 1970 JQSQBRUMBELOW I DUAL CAVI-TY BANDBASS' FILTER Filed Spt19) 1967 2 SheetS- Shet 1 FIG.

INVENTOR. JOSEPH s. BRUMBELOW' June 2, 1970 I "is.la r e' ulvl zflzLcivVv ,515 50 DuAnb vITY BANDBASSYFVILTBR Filed Sept. 19, 19s? I 1 2Sheet-Sheet 2 I INVENTOR.

JOSEPH s. BRUMBELOW United States Patent ()ffice 3,516,030 Patented June2, 1970 3,516,030 DUAL CAVITY BANDPASS FILTER Joseph S. Brumbelow, P.O.Box 63, Newton, Mass. 02160 Filed Sept. 19, 1967, Ser. No. 668,810 Int.Cl. H03h 7/10, 9/00 U.S. Cl. 333-73 2 Claims ABSTRACT OF THE DISCLOSUREThis invention concerns apparatus for filtering electromagnetic waveenergy according to the frequency of its vibrations and moreparticularly relates to a bandpass filter employing dual resonantcavities.

Bandpass filters are devices which transmit electromagnetic waves whosefrequencies lie within a continuous region (i.e., the band) of theelectromagnetic spectrum of frequencies and reject those waves whosefrequencies are outside the band. In the ideal bandpass filter, theelectromagnetic energy whose frequencies are within the band proceedsthrough the filter without attenuation whereas the energy at all otherfrequencies is totally prevented from passing through the filter.

It is well known that filters can be constructed of lumped elements,that is, of elements such as coils and condensers, whose reactance isprincipally of one kind, that is, whose reactance is principallyinductive or capacitive. At higher frequencies, filters are constructedof resonators formed by semi-lumped elements or from elements whosecapacitance and inductance are distributed, such as coaxial lineresonators, strip transmission line resonators, or waveguide resonators.Hollow waveguide or cavity resonators usually provide the best unloadedQs and, therefore, are used for filters where low insertion loss and abandwidth in the microwave region are desired. However, waveguideresonators have the disadvantage of being bulky. It is the principalobject of the invention to provide a filter employing cavity resonators,that is, materially less bulky than waveguide resonator filters ofconventional construction.

The invention resides in a pair of cavity resonators that are arrangedside by side and are coupled together by a series resonant configurationthrough their common wall. Each cavity is provided with a post that ispositioned longitudinally within the cavity and is attached at one endto the cavitys end wall. The free end of the post has a flat face whichconfronts the opposite end of the cavity to produce a condition ofresonance within the cavity. In one embodiment of the invention, thelongitudinal posts are adjustable in length to permit tuning of thecavities. In another embodiment the length of the posts are fixed andtuning screws are provided in each cavity. A marked fore-shortening ofeach cavity is brought about by attaching to the free end of the post adisc which is materially larger in diameter than the diameter of thepost. By coupling to a selected point along the length of the post inone resonator, the input impedance can be matched to the impedance ofthe signal source. Similarly, the output impedance of the filter can bematched to the impedance of the output circuit by coupling to a selectedpoint on the post in the other cavity.

The invention, both as to its construction and manner of operation canbe better understood from the following exposition when considered inconjunction with the accompanying drawings, in which:

FIG. 1 is a sectional view of a preferred embodiment of the invention;

FIG. 2 is a sectional view of an embodiment of the invention in whichthe length of the posts are fixed and tuning screws are provided in thecavities; and the coupling between cavities is fixed;

FIG. 3 depicts a lumped element equivalent of the invention;

FIG. 4 is an enlarged view of a portion of the invention and depicts asecond embodiment of the coupling arrangement between the cavities;

FIG. 5 is a sectional view of the invention and depicts a thirdembodiment of the coupling arrangement between the cavities.

Referring now to FIG. 1, there is shown a pair of similar hollowcylindrical cavity resonators 1 and 2 connected side to side and havinga common wall 3. The common wall has an opening or iris 4 in it. Theopposite end of resonator 1 is closed by a plate 5 having a sleeve 6extending into the cylinder along the cylinders longitudinal axis. Apost 7, having a screw-threaded shaft 8 mating with threads in the endplate 5, or in sleeve 6, extends through the sleeve and has a disc 9attached to its free end. The other end of post 7 is slotted and as theshaft is engaged with the threads of the end plate 5, by turning theshaft, the post can be advanced in to the cavity or withdrawn from it. Asource of trouble in the filter can result from the failure of the postto maintain a good electrical connection with sleeve in which it slides.It is preferred, therefore, to construct the sleeve 6 with fingers whichgrip the post and yet permit the post to move to and fro in the sleeve.The fingers may be formed by slotting the free end of the sleeve anddeforming the fingers inwardly so that they are spread by the insertionof the post.

In cavity 2, is a similar disc 10 that is attached to the free end ofthe post 11 which is slidably mounted in a sleeve 12 attached to endplate 5. The plate 5 closes off the end of resonator 2 and the plate isprovided with threads 14 which engage the threaded shaft of post 11 andpermit that post to be advanced into or retracted from the cavity.

To provide signal coupling into and out of the filter; the outerconductor of a coaxial connector 15 is secured to the wall of resonator1 and the center conductor is connected to the sleeve 6 at a distance mfrom end plate 5. Similarly, a second coaxial connector 16 is secured tothe wall of resonator 2 and sleeve 12.

FIG. 3 is a diagrammatic representation of the lumped constantequivalent of FIG. 1 embodiment. Cavity reso nator 1 is represented byinductance L and capacitance C cavity resonator 2 is represented byinductance L and capacitance C and the coupling between the resonatorsis represented by capacitance C C and inductance L A considerable partof the inductance L resides in the sleeve 6 and post 7 and thecapacitance C if formed by the capacity between disc 9 and the end Wall17. Similarly, for cavity resonator 2, a part of inductance L resides inthe sleeve 12 and post 11, and the capacitance C is formed, by thecapacity between the end wall 17 and disc 10.

C is formed 'by the capacitance between the sleeve 6 and the rod 20, Cis formed by the capacitance between sleeve 12 and rod 20. Aconsiderable part of the inductance L resides in the rod 20. C C and Lform a series resonant condition to couple the desired signal fromcavity 1 to cavity 2. Rod 20 passes through an opening 4 from cavity 1to cavity 2. If the length B of post 7 is altered, the inductance L willbe altered and the resultant change in position of disc 9 will causecapacitance C; to change. The resonant frequency of cavity 1 can betuned by adjusting the length B of post 7. Similarly, the resonantfrequency of cavity 2 can be tuned by adjusting the length of post 11.Dielectric material 18 and 19 insulate and position rod 20. Anonmetallic screw 21 allows rod 20 to be positioned along sleeves 6 and12 to change the amount of coupling between cavities 1 and 2. Thedimensions d and l of rod 20 determine the inductance, L and withdielectric 18 and 19 of thickness q determine the capacitance of C and CDiscs 9 and 10 should be of sufficient thickness if so as to be rigiddespite the thermal and electrical stresses set up in those members.Beyond the requirement of rigidity, the 10 disc thickness can beincreased to enhance somewhat the capacitance that is a parameter of theresonant cavity.

It is assumed that both cavity resonators, 1 and 2 are tuned to the samefrequency, although it is evident that those resonators may be tuned toresonate at different frequencies, if such operation were required. Thelength B of the post in the FIG. 1 embodiment is approximately equal toA/ 8 where A is the wavelength in free space of a wave vibrating at themid-band frequency of the filter.

The characteristic impedance of the cavities can be assumed to be givenby I) Z0 10g a where b is the internal diameter of the cavity a is theexternal diameter of sleeve 6 e is the relative dielectric constant ofthe medium in the cavity.

Because the length B of the post in the cavity is inductive, the post isequivalent to the winding of a transformer. The distance m, measuredfrom the end plate to a point on the post or on the sleeve, determinesthe input impedance of the cavity 1. Similarly, the distance measuredbetween corresponding points in cavity 2 determines the output impedanceof that cavity. Since the dual coupled cavities form a symmetricaldevice, the input signal may be impressed upon either connector 15 or 16and the output of the filter is obtained from the other connector. The 9impedance of most coaxial connectors has been standardized at 50 ohmsand when such standard connectors are employed, the distance m isselected sothat the center conductor is connected to give the bestimpedance match.

FIG. 2 depicts an embodiment of the invention in which the posts 28 and29 are fixed in length rather than being of adjustable length as in theFIG. 1 embodiment. To permit tuning of the cavity resonators 30 and 31,a tuning screw is provided in each resonator. The tuning screw 24 islocated in the wall of resonator 30 in a position where it is adjacentto disc 25. Similarly, the tuning screw 26 in the wall of resonator 31is in a position where the screw is adjacent to disc 27. By adjustmentof screw 24, the cavity resonator 30 can be tuned and by adjustment ofscrew 26 the other resonator can be tuned. However, the tuning of oneresonator affects the other so that each cavity cannot be tunedcompletely independently of the other. In other respect, the embodimentof FIG. 2 is similar in construction to the FIG. 1 embodiment.

In the embodiments of FIGS. 1 and 2, the longitudinal 6O axial length ofthe resonators has been markedly foreshortened by employing discs on thefree ends of the posts. Where an increase in the bulk of the dual cavityfilter can be tolerated, posts 28 and 29 can he used without end plates.The elimination of the end plates necessarily requires a cavity ofgreater length to maintain the same resonant frequency. The cavity isequivalent to an open circuited quarter wave resonant line and thereforethe cavity, without the end plates, is approximately a quarterwavelength at the mean wavelength of the filter.

FIG. 4 depicts a modification of the invention which permits a greaterrange of adjustment in the coupling between the two cavities. Theinsulative sleeves 33, 34 of the FIG. 4 embodiment correspond to thesleeves 18 and 19 of the FIG. 1 embodiment except that the sleeves 33,34 cover a larger longitudinal extent of the posts. The ends of thecoupling rod 32 partially encircle the sleeves 33, 34 and are able toslide along those sleeves when the screw is turned to raise or lower thecoupling rod. As the sleeves extend below the points where the input andoutput connections tap onto the posts, apertures are provided in thesleeves through which the connections to the posts are made. The ends ofrod 32, because they do not completely encircle the sleeves, permit therod to be lowered without interference from the input and outputconnections.

FIG. 5 depicts a further embodiment of the invention in which thecoupling rod 35 is fixed in position. Each of the posts has a transverseaperture lined with an insulative sleeve 36 or 37. The coupling rodbridges the distance between the two posts and has its ends disposed inhe sleeves 36, 37.

Because variations in embodiments of the invention are possible in thelight of the foregoing teaching, it is intended that the invention notbe limited to the details of construction and arrangement of partsspecifically described or illustrated. Rather, it is intended that thescope of the invention be delimited by the appended claims and encompasssuch structures as do not in essence depart from the invention theredefined.

What is claimed is: 1

1. A band pass filter comprising a pair of resonant cavities disposedside by side and having between them a common wall, the common wallhaving an aperture extending between the two cavities, each cavity beingterminated by an end wall, each end wall having a conductive postprotruding therefrom into the cavity, each post having a flat surface atits free end, confronting the opposite end wall of the cavity, the postsin the two cavities being parallel, means for coupling a signal into onecavity, means for extracting a signal from the other cavity, aconductive coupling rod extending through the aperture in the commonwall transversely to the parallel posts, the ends of the coupling rodbeing proximate to the posts to capacitively couple the rod to theposts, dielectric members interposed between the ends of the couplingrod and the adjacent posts, and the coupling rod being of a length whichin conjunction with its capacitive coupling provides a low impedancepath between the cavities for signals in the filters pass band.

2. The invention according to claim 1, further including means formoving the coupling rod to cause the ends of that rod to be shiftedlongitudinally along the posts.

References Cited UNITED STATES PATENTS 7/1959 Farmer.

OTHER REFERENCES HERMAN KARL SAALBACH, Primary Examiner L. ALLAHUT,Assistant Examiner US. Cl. X.R. 33383

